Musculoskeletal tissues contain a very small and heterogeneous populations of stem cells and progenitor cells that contribute constantly to the repair and remodeling of connective tissues. We have defined and used the inclusive term Connective Tissue Progenitors (CTPs) for these cells. CTPs are also central to efforts in Orthopaedic Tissue Engineering, both as tools for cell therapy and as targets for local and systemic therapy. Optimal use of CTPs will require knowledge of (and sometimes control of) the concentration, prevalence and biologic characteristics of the CTPs in the site of treatment. It also requires minimizing the risk and effort needed for the harvest of CTPs. The broad aim of this proposal is to contribute to the field of tissue engineering and bone biology by providing a quantitative characterization of the epidemiology and kinetics of human CTPs in cancellous bone and three relevant alternative clinical sources of CTPs (aspirated bone marrow, fat tissue, and muscle). We also hope to contribute through further development and exploration stem cell kinetics in human cancellous bone in the context of a mathematical model linking stem cell kinetics with classic histomorphometry assessment of bone remodeling. Cores of cancellous bone will be harvested from 160 patients undergoing elective hip or knee replacement for osteoarthritis. Marrow aspirates, skeletal muscle and subcutaneous fat will also be collected. Using a unique set of quantitative tools developed in our laboratory, the concentration, prevalence and biologic performance of the CTPs in each sample will be measured using colony specific assays. Microcomputed tomography and quantitative histomorphometry will be used to define the structural, histologic and dynamic remodeling environment in the local bone. These data will be combined to define the relationship between CTP epidemiology and kinetics in the marrow space and on the trabecular surface, and structural, histologic and dynamic bone remodeling activity. These data will provide surgeons and tissue engineers, for the first time, with an objective comparison of the concentration, prevalence and biologic potential of the CTPs in relevant human tissues. This will enable the rational design of strategies for the harvest, concentration, selection, transplantation or targeting of CTPs in human tissues. The strategy proposed will also provide a quantitative platform of tools and information upon which we and other investigators can build a more robust understanding of the human connective tissue stem cell system. These data will enable exploration and development and assessment of a quantitative theoretical model of the kinetics of stem cell activation and CTP kinetics in skeletal remodeling that is linked to classic histomorphometric parameters.